Perch Members - Login with Username

I have a client that would like their staff to be able to login with a username instead of an email address (like the Perch CMS login) using the Members app. Is this a possibility?

After adding a new text input called "username" into the /members/member.html template and modifying the login_form.html template to match that same input name, I can't get Perch to recognize it. When I use <perch:showall> on the login_form.html, there isn't much to go on, so I'm not sure what I have available to me.